Pressure/temperature/substitution-induced melting of A-site charge disproportionation in Bi1-xLaxNiO3 (0≤x≤0.5)

Metal-insulator transitions strongly coupled with lattice were found in Bi1-xLaxNiO3. Synchrotron x-ray powder diffraction revealed that pressure (P∼3GPa, T=300K), temperature (T∼340K, x=0.05), and La substitution (x∼0.075, T=300K) caused the similar structural change from a triclinic (insulating) to an orthorhombic (metallic) symmetry, suggesting melting of the A-site charge disproportionation. Comparing crystal structure and physical properties with the other ANiO3 series, an electronic state of the metallic phase can be described as [A3+L δ, Ni2+L 1-δ], where a ligand-hole L contributes to a conductivity. We depicted a schematic P-T phase diagram of BiNiO3 including a critical point (3 GPa, 300 K) and an inhomogeneous region, which implies universality of ligand-hole dynamics in ANiO3 (A=Bi, Pr, Nd...).